This invention relates to a blood pressure measuring apparatus for continuously measuring a blood pressure in a noninvasive mode, and more particularly to a continuous noninvasive blood pressure measuring apparatus having means for compensating a cuff pressure which varies with the temperature of the air in the cuff.
In order to monitor an arterial pressure for a long period of time, there are available two methods. One of the methods is an invasive blood pressure measuring method which is practiced with a pressure transmitting tube or pressure sensor inserted into the blood vessel, and the other is a noninvasive blood pressure measuring method in which a sensor is set outside the body under examination; that is, the living body is not hurt at all. In one example of the noninvasive blood pressure measuring method, the diameter of a blood vessel, which varies with the pressure in it, is measured, and the blood pressure is calculated from the diameter thus measured. Hereinafter, the noninvasive blood pressure measuring method in which a blood pressure is measured from the variation in diameter of a blood vessel, will be described.
In the noninvasive blood pressure measuring method, with a cuff put on the brachium (or upper arm), a mean arterial pressure, a systolic pressure, and a diastolic pressure are measured according to the ordinary oscillometric method, and those pressures are employed calibrating values. Thereafter, the cuff pressure is set to a pressure, for instance 20 mmHg, lower than the diastolic pressure, and then the cuff is closed. Under this condition, the following coefficients are determined: First, a coefficient is obtained which is used for allowing the D.C. component of the cuff pressure provided immediately after the closing of the cuff to correspond to the mean arterial pressure. Next, several pulses provided through the cuff are detected and averaged to form a pulse waveform, and a coefficient is obtained which is used for allowing the peak value of the pulse waveform thus formed to correspond to the systolic pressure, and similarly a coefficient is obtained which is used for allowing the bottom value of the pulse wave to correspond to the diastolic pressure. The D.C. variation of the cuff pressure reflects variations in mean arterial pressure, and the cuff pulse waveform reflects variations both in systolic pressure and in diastolic pressure. Hence, the arterial pressure waveform can be continuously calculated by using the D.C. variation in pressure of the closed cuff, the measured values of the cuff pulse waveform, and the coefficients thereof.
In the above-described conventional noninvasive blood pressure measuring method, the cuff is closed, so that the cuff pressure changes with the temperature of the air in the cuff. Therefore, it is difficult to determine whether the variation in cuff pressure is due to the variation in mean arterial pressure or whether it is due to the variation in cuff temperature. As a result, sometimes the blood pressure calculated includes a serious error.
Heretofore, this diffuculty is eliminated as follows: When the blood pressure calculated is deviated by a certain value from the value which is obtained by the initial calibration measurement, then the calibration measurement is carried out again to renew the mean arterial pressure, systolic pressure and diastolic pressure thereby to calculate new coefficients.
Hence, immediately after the blood pressure measuring operation starts; that is, when the cuff temperature is abruptly raised by the body temperature of a person under examination, it is necessary to perform the calibration measurement frequently (for instance every three to five minutes). This will displease him.